Apsidal motion in the massive binary HD 152248

Constraining the internal structure of the stars

¹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août, 19c, Bât B5c, 4000 Liège, Belgium
e-mail: sophie.rosu@uliege.be
² Observatoire astronomique de l’Université de Genève, Maillettes 51 – Sauverny, 1290 Versoix, Switzerland

Received: 8 May 2020
Accepted: 28 August 2020

Abstract

Context. Apsidal motion in massive eccentric binaries offers precious information about the internal structure of the stars. This is especially true for twin binaries consisting of two nearly identical stars.

Aims. We make use of the tidally induced apsidal motion in the twin binary HD 152248 to infer constraints on the internal structure of the O7.5 III-II stars composing this system.

Methods. We build stellar evolution models with the code Clés assuming different prescriptions for the internal mixing occurring inside the stars. We identify the models that best reproduce the observationally determined present-day properties of the components of HD 152248, as well as their internal structure constants, and the apsidal motion rate of the system. We analyse the impact on the results of some poorly constrained input parameters in the models, including overshooting, turbulent diffusion, and metallicity. We further build “single” and “binary” GENEC models that account for stellar rotation to investigate the impacts of binarity and rotation. We discuss some effects that could bias our interpretation of the apsidal motion in terms of the internal structure constant.

Results. The analysis of the Clés models reveals that reproducing the observed k₂ value and rate of apsidal motion simultaneously with the other stellar parameters requires a significant amount of internal mixing (either turbulent diffusion, overshooting, or rotational mixing) or enhanced mass-loss. The results obtained with the GENEC models suggest that a single-star evolution model is sufficient to describe the physics inside this binary system. We suggest that, qualitatively, the high turbulent diffusion required to reproduce the observations could be partly attributed to stellar rotation. We show that higher-order terms in the apsidal motion are negligible. Only a very severe misalignment of the rotation axes with respect to the normal to the orbital plane could significantly impact the rate of apsidal motion, but such a high misalignment is highly unlikely in such a binary system.

Conclusions. We infer an age estimate of 5.15 ± 0.13 Myr for the binary system and initial masses of 32.8 ± 0.6 M_⊙ for both stars.